This invention relates to power-supply apparatus in which the power is derived from the energy generated in a nuclear reactor. Such apparatus is sometimes referred to as a nuclear-reactor power-supply plant. This invention has particular relationship to such apparatus in which there are integrated facilities for preventing the reactor from operating when off-normal conditions occur in the apparatus. The invention was created to deal with the operation of these facilities as part of the power supply apparatus.
In the interest of facilitating the understanding of the invention by reference to concrete concepts, this application confines itself predominantly to a powersupply in which the thermal energy is developed by a pressurized-water reactor (PWR). This invention is applicable to reactors of other types including boiling-water reactors (BWR), liquid-metal coolant and gas-coolant reactors and the application of this invention or its principles to such other reactors is intended to be within the scope of this invention and of any patent which will issue on or as a result of this application.
The power-supply apparatus with which this invention concerns itself includes in addition to the nuclear reactor, components for converting the thermal energy generated by the reactor into electrical energy. Typically, the principal such components for a pressurized water reactor are the pressurizer, the steam generators, the turbine, the electrical generator, the condenser and their associated parts.
The nuclear reactor and these components, while in operation, have processes each of which has one or more paraeters. There may be as many as 18 or more of these parameters. The most notable among the parameters are the neutron flux, the power generated by the reactor, the reactor-coolant temperatures, the pressurizer water level and pressure, the steam-generator feedwater level and the pressure of its steam, the feedwater flow and others. There may also be categorical parameters, such as the status of a switch, whether it is closed or open, or the status of a pump, whether it is on or off. It is necessary that these parameters be monitored reliably and for this purpose a plurality of like sensors are provided for monitoring each parameter. The use of more than one sensor for each parameter is a redundancy but this practice of redundancy is adopted in the interest of achieving optimum reliability. The aggregate of like sensors which monitor each parameter are referred to in this application as a set or sensors.
Generally, it may be assumed that there are n sensors in a set. When a sensor is being tested or is subject to maintenance or is disabled, it is taken out of service, a sensor which is taken out of service in this way is said to be bypassed. In case of a bypass, only unbypassed sensors for the parameter involved, i.e., of the set involved, are available to monitor this parameter. When the sensors in service sense an off-normal parameter, the control rods are inserted fully into the core of the reactor to prevent the reactor from supplying thermal power by reducing substantially its neutron flux. Within the scope of this invention, the neutron flux may be reduced by increasing the shim in addition to inserting rods or by increasing the shim alone adequately. This operation is referred to here as tripping the reactor or as tripping the power-supply apparatus. This application also refers to the tripping of a circuit breaker. Reference to the tripping of a breaker should not be taken to mean that a reactor is tripped. In practice, a reactor is not tripped if one sensor of a set of like sensors, senses an off-normal parameter because it is undesirable that tripping take place for spurious or transient off-normal sensing by a sensor. Such spurious tripping would unreasonably increase the down time. Generally, it may be assumed that an off-normal parameter must be sensed by at least m unbypassed like sensors of a set of n sensors (n&gt;m) to produce a trip if the number of unbypassed sensors is greater than m. If the number of unbypassed sensors of a set is less than or equal to m, the off-normal parameter must be sensed, by the numnber of unbypassed sensors less 1 to produce a trip. The number m may be a constant or it may vary as sensors are taken out of service.
Typically, n is 4 and m is 2. If the number of unbypassed-like sensors is 2, a trip is produced by sensing of an off-normal parameter by only 1 sensor and if the number of unbypassed sensors is 1 there is an automatic trip.
The sensors and the cooperative elements which respond to the signals which the sensors sense are grouped in trip-logic channels. The grouping may be structural but it may also be functional with the actual components which carry out the functions intermingled. There are as many channels as there are sensors in each set of sensors, i.e., generally n channels. Each channel usually contains one each of the unlike sensors which sense the different parameters. Typically, there are four channels.
The reactor is tripped by opening or tripping circuit breakers. There are twice as many breakers as there are sensors per parameter, i.e., generally 2 n breakers. The contactors of the breakers are so interconnected that 2 m contactors must be opened to trip the reactor. Typically, there are four sensors per parameter, and four channels. There are eight breakers, two controlled by each channel, and the contactors of the breakers are so connected that four contactors must opened or tripped to trip the reactor. The action which takes place when one sensor of a set senses an off-normal parameter, is here called a partial trip of the reactor.
In the interest of facilitating the understanding of this invention, the following description of this invention will be confined to apparatus in which there are sets of four sensors per parameter and accordingly four channels and the reactor trip is actuated on a "two out four configuration". It is to be understood that the use of sets of more than four sensors per parameter and a different reactor trip configuration is within the scope of this invention.
When one of the sensors of a set is bypassed, it is necessary that this bypassing shall not result in a partial trip status of the reactor. If this occurred, the sensing of an off-normal parameter by another sensor of the set, i.e., by only one sensor, would trip the reactor.
It is desirable that the reliability of the sensing and tripping for the reactor be preserved without spurious tripping of the reactor notwithstanding that one or more sensors are bypassed. It is an object of this invention to preserve this reliability while avoiding spurious trips in the face of bypasses.
More generally stated, it is an object of this invention to provide for the monitoring of the parameters of a nuclear-reactor power-supply system with the reliability demanded by the nuclear-power art while minimizing the down-time of the reactor.
It has been suggested that a partial trip resulting from a bypass be precluded by closing additional breakers across the breakers in whose channel a sensor has been bypassed. Such an arrangement would require eight auxiliary breakers and would be complicated, costly to install and costly to maintain.
It is also an object of this invention to overcome the above-described drawback and to prevent the bypassing of a parameter sensor in nuclear-reactor, power-supply apparatus from having the same effect as a partial trip of the reactor in an uncomplicated manner at a lower installation and maintenance cost. It is also an object of this invention to provide nuclear-reactor, power-supply apparatus including uncomplicated, low-cost facilities maintainable at low cost, for preventing the bypassing of a parameter sensor from having the effect of a partial trip of the reactor and it is a further object of this invention to provide a method for operating such apparatus.
More generally, it is an object of this invention to provide nuclear-reactor power-supply apparatus in which the parameters shall be monitored with the reliability demanded by the nuclear art and which shall meet the above objects.